For "relic" meaning an object from the past:
* Modernity
* New
* Contemporary
* Up-to-date
* Current
For "relic" meaning something outdated or obsolete:
* Modern
* Advanced
* Cutting-edge
* Upgraded
* State-of-the-art
For "relic" meaning something that is sacred or venerated:
* Profane
* Unholy
* Desecrated
* Common
* Mundane
The best antonym will depend on the specific way "relic" is being used in the sentence.